Abstract:

The agricultural production of peppermint has been shown to contribute significant quantities of nitrate-nitrogen to groundwater recharge. In an effort to provide new tools for increasing nitrogen efficiency within peppermint production, three research questions were proposed: i) How should plant tissue samples be collected to achieve the greatest precision when using the mint stem nitrate test for nitrogen management?; ii) What is the consumptive use of water by peppermint in the post-harvest period?; and iii) How does irrigation uniformity affect nitrate loading to groundwater when N is supplied through chemigation?
In the first investigation, structured field experiments were designed and conducted on commercial peppermint fields to isolate potential environmental, management, and sampling influences on stem nitrate test results. The most significant effects observed were those of the type of stem material collected (a 441% effect at p<0.001) and the number of stems collected to estimate the field mean concentration. It was found that the variance of the sample population and the number of stems required for a given sampling error could be greatly reduced by only collecting stems from within the plant canopy. Less pronounced but statistically significant differences in stem nitrate concentrations were produced by variations in solar radiation on hourly (a 17% effect at p<0.05) and daily (a 29% effect at p<0.01) scales. In an analysis of stem nitrate spatial variability, a purely random distribution of stem nitrate concentrations was observed on the 1-150 m scale.
For the second investigation, a field study was conducted to measure the consumptive use of peppermint in the post-harvest period and to develop crop coefficients (Kc) used to predict evapotranspiration rates. The soil water balance was measured on two fields with a neutron moisture probe over an 80 day period. Over the 49 days following harvest, a cumulative consumptive use of 96 mm was observed. Basal crop coefficients increased from near zero to approximately 0.40 within 40 days post-harvest.
The third, and final, investigation developed a simple heuristic statistical model to explore the effective adequacy of chemical application as influenced by the uniformity of irrigation. To perform this analysis, an expression was presented whereby irrigation distribution parameters for the normal, or Gaussian, model could be derived from common irrigation design terms. The results of this model indicate that the effective chemical adequacy is greatly compromised when the irrigation uniformity coefficient is low and/or the design irrigation adequacy is high.